Purpose/Significance of Application:To evaluate long-term performance and hydraulic conductivity of PRBs in different hydrogeologic settings and with varying measurement techniques

Regulatory Requirements/Cleanup Goals:
The objectives for the demonstration were to evaluate longevity and hydraulic performance of PRBs at sites with varying hydogeologic conditions; no specific cleanup goals were identified

Results:- Longevity was evaluated at Moffett Field and Lowry AFB using accelerated column tests, looking at changes in the reaction rates (half lives) of contaminants when the iron was exposed to many pore volumes of contaminated groundwater; actual groundwater from the two sites was used
- The PRBs at Moffett and Lowry had projected lifetimes of approximately 30 years, based on the time period over which the reactivity of the iron would decline by a factor of two
- Hydraulic performance was evaluated at all four sites based on a characterization of the flow regime around existing field barriers, and on results from groundwater modelling of the factors that determine flow, looking at groundwater capture zones and residence times
- The hydraulic evaluation showed that careful water level measurements combined with groundwater modeling provided the best results, sometimes contrasting with results from direct flow measurements

Cost Factors:- A present value analysis of the projected costs for a PRB and comparable pump and treat system showed that it would take approximately 7 to 10 years to obtain a payback on the initial capital investment in a PRB
- Cost for field demonstration of PRB at Moffett was reported as $802,375, consisting of $652,375 in capital costs and $150,000 in O&M costs (based on 5 monitoring events)
- Cost for field demonstration of PRB at Dover was reported as $739,000, consisting of $365,000 for pre-construction activities and $374,000 for PRB construction

Description:Naval Air Station (NAS) Moffett Field and Lowry Air Force Base (AFB) are former DoD operational facilities that have groundwater contaminated with halogenated volatile compounds. Both sites had installed PRBs and had historical information available about field operation of the PRBs. The sites were evaluated to determine expected longevity of the PRBs, including groundwater geochemistry monitoring, iron core collection and analysis, geochemical modeling, and accelerated column tests. These tests showed that the reactivity of the iron declined with long-term exposure to groundwater and was related to the concentration of dissolved solids and rate of groundwater flow through the reactive media.

Hydraulic performance was evaluated for PRBs at Moffett Field, Lowry AFB, Seneca Army Depot, and Dover AFB. Moffett, Lowry, and Dover used funnel and gate configurations, while Seneca used a continuous trench design. The hydraulic performance was evaluated using water level measurements and slug tests, HydroTechnicsTM flow sensors and colloidal borescope, and groundwater flow and solute transport modeling. Results showed that careful water level measurements combined with groundwater modeling provided the best results for hydraulic performance. These results sometimes contrasted with the results from direct flow measurements, possibly due to localized differences in groundwater flow conditions.